WASHINGTON: US President Donald Trump on Monday fired the acting attorney general, a holdover from the Obama administration, after she ordered Justice Department attorneys not to defend his controversial immigration orders.

In a sharply worded statement, the White House called Ms Sally Yates "weak on borders and very weak on illegal immigration" and also criticised Democrats for not yet confirming the appointment of attorney general-designate Jeff Sessions.

"The acting attorney general, Sally Yates, has betrayed the Department of Justice by refusing to enforce a legal order designed to protect the citizens of the United States," the White House said in a statement.

"This order was approved as to form and legality by the Department of Justice Office of Legal Counsel," it said.

"Tonight, President Trump relieved Ms Yates of her duties."

Federal prosecutor Dana Boente will serve as acting attorney general "until Senator Jeff Sessions is finally confirmed by the Senate, where he is being wrongly held up by Democrat senators for strictly political reasons," it said.

With Mr Trump's White House facing multiple lawsuits and worldwide criticism over an order banning migrants from seven Muslim nations, Ms Yates had whipped the rug from under her boss in a defiant and damaging parting shot.

In a memo to Department of Justice staff, Ms Yates - a career government lawyer promoted by Mr Barack Obama - expressed doubts about the legality and morality of Mr Trump's decree, which has prompted mass protests.

NOT CONVINCED

"My responsibility is to ensure that the position of the Department of Justice is not only legally defensible, but is informed by our best view of what the law is," she wrote.

"I am not convinced that the defence of the executive order is consistent with these responsibilities nor am I convinced that the executive order is lawful."

It was a remarkable act of defiance against a tough-talking president who has shown little sign of brooking insubordination.